In trees, the heartwood (duramen) is the dead, denser wood towards the core of the tree where sap doesn't flow, but where the majority of the tree's structure comes from. Conversely, the sapwood (alburnum) surrounds the heartwood, and is the living part of the tree where nutrients flow between the roots and leaves. All wood starts out as sapwood, but over time the inner portions which are not needed to pass sap change to heartwood to provide further reinforcement.

For example, the classic yew longbow was cut so that the natural divide between heartwood and sapwood would pass down the length of the bow stave. The heartwood was used in the belly of the bow, since it resists compression, while the sapwood was used on the back, since it is more stretchy. That way, you get a 'composite' effect from a single span of wood.

Don't think it has anything to do with any magical material.

"...I know that kind of man / It's hard to hold the hand of anyone / Who is reaching for the sky just to surrender..."– Leonard Cohen, The Stranger Song

Duskwood and Zalantar are different types of tree wood, much like ash, yew, oak, and pine would likewise have different properties. Heartwood and sapwood are different regions within the same tree - like I mentioned with yew, there is yew heartwood and yew sapwood which play different roles in the carving of a longbow.

"...I know that kind of man / It's hard to hold the hand of anyone / Who is reaching for the sky just to surrender..."– Leonard Cohen, The Stranger Song